Silicon carbide is the only compound species in the Si-C system, but it can occur in many polytype structures. The lone cubic polytype crystallizes in the zinc blended structure and is denoted "Beta-SiC." The approximately one hundred seventy known additional hexagonal and rhombohedral polytypes are collectively referred to as "Alpha-SiC."
Beta-SiC has been long considered as an excellent candidate semiconductor material for high-temperature, high-power and high-speed device applications due to its unique physical and electronic properties of high melting point, high thermal conductivity, wide band gap, high breakdown electric field and high saturated electron drift velocity. The high thermal conductivity and breakdown field also indicate that high device densities can be achieved utilizing Beta-SiC. Unfortunately, early research on the use of Beta-Sic as a semiconductor was hindered by the difficulty of the reproducible growth of high quality crystals.
In recent years Beta-SiC has been actively studied because crack-free, reproducible monocrystalline Beta-SiC films have been obtained on Si(100) substrates by chemical vapor deposition (CVD). However, analysis of these films by means such as cross-sectional transmission electron microscopy (XTEM) has revealed a high density of defects, mainly stacking faults and the partial dislocations that bound them, at the Si substrate/Beta-SiC interface. A majority of these defects extend at least 3 .mu.m into the film, but many extended to the as-grown surface (up to 20 .mu.m). These defects occurred primarily as a result of the lattice mismatch (.about.20%) and the thermal expansion mismatch (.about.8%) between the two materials. They cause unacceptable charge carrier redistribution and the introduction via diffusion of impurity dopants from the Beta-SiC film into the Si substrate. It is also probable that these defects reduce the carrier mobilities and lead to large leakage currents. Thus, it is clear that it would be very advantageous to find a substrate more closely matched to SiC in both lattice parameter and thermal expansion than Si.
The growth of SiC on SiC substrates via chemical vapor deposition has been reported since the late 1960's. Several researchers have reported on the growth of Alpha-SiC on Alpha-SiC substrates in the temperature ranges 1500.degree.-1760.degree. C. and 1320.degree.-1390.degree. C. with growth directions parallel to the [0001] axis and perpendicular to the [0001] axis, respectively. A few researchers also reported the growth of extremely thin Beta-SiC films on Alpha-SiC substrates in the temperature range of 1500.degree.-1700.degree. C. and the growth of Beta-SiC on Beta-SiC substrates in the temperature range of 1200.degree.-1800.degree. C. In some cases, the investigators reported the achievement of single-crystal Beta-SiC epilayers. The SiC epilayers and the SiC substrates were not analyzed by techniques capable of noting the defects that were, in fact, formed in the bulk thin films, or at the interfaces Nor were studies undertaken to establish the utility of these films for commercial device applications.